Necessity of GPUs and hardware minimum specs for Deep Learning?
For a simulation of SNES or N64, the resolution is probably not that high. You can use either online credits or buy new hardware. For online credits, it is recommended if you do teh simulation/training for only several dozens hours, as eache hour costs around 5-10 dollars. AWS is a good choice as it have a large variety of choice for hardware. For higher performance, choose multi GPU setup.

However, if the simulation requires a long time, or you wish to own the hardware, you should buy your own hardware. Multiple GPU will scale quite nicely, so you may go for multi GPU if performance is needed. For single GPU, a RTX 2000 series is recommended as it have tensor cores to accelerate training of CNN. For a budget option, consider an RTX 2060 SUPER. It have 8 GB of VRAM with a relatively low price and yet it have tensor core functionality. For more powerful GPU, choose the RTX 2080 TI. It have 11GB of VRAM which is enough in most use case. 
For multiple GPU, you might want several RTX 2060 SUPER as they are cheap but the performance is quite good. The 8GB VRAM helps as well. For the highest performance, choose 2 or more RTX 2080 TI. It is enough for nearly most task. If this is not enough, renting an online instance is a better idea. Hope it helps and have a nice day.